Sound pick-up device



March 13, 1934. H o s -r' Re. 19,115

SOUND PICK-UP DEVICE o iginal Filed May 20. 1932 :s Sheds-Sheet 1 Jkvzmons HARRY .s OLSON An NEY March l3, 1934.

H. F. OLSON ET AL Reissued Mar. 13, 1934 SOUND on-Ur DEVICE Harry F. Olson, Collingswood, and Julius Weinberger, Haddonfleld, N. 1., assignors to Radio Corporation of America, a corporation of Delaware Original No. 1,892,645, dated December 27, 1932, Serial No. 612,466, May 20, 1932. Application for reissue April 4, 1933, Serial No. 664,441

8 Claims. (Cl. 179-438) Our invention relates to sound pickup devices such as ribbon microphones, and has for its principal object the provision of an improved pickup device which is capable of collecting sound from a; predetermined range of directions and excluding sound not originating within this range.

A further object is the provision'of a microphone which responds both to the pressure com-- ponent and to the velocity or pressure gradient component of the sound wave.

Referring to the drawings, Fig. 1 isa front view of a microphone constructed in accordance with our invention,

Figs. 2, 3, and 4 are, respectively, side, back 1 and bottom views of this microphone,

the microphone,

Figs. 6 to 8 illustrate diiierent characteristics of a velocity or pressure gradient microphone, Figs. 9 to 11 illustrate similar characteristics of a microphone responsive to the pressure component of a sound wave,

Fig. 12 illustrates the directional characteristic of the pressure gradient microphone, the pressure microphone and the microphone illustrated by Figures 1 to 5, and

Fig. 13 illustrates the observed directional characteristics of the improved microphone.

In order that sound radiation may be projected from one point to another point or area with a maximum of efliciency and a minimum o interference from reflecting surfaces directional sound radiators have been almost universally employed for sound sources in large scale reproduction of sound. A similar directivity has been'found to be desirable in the sound pickup system to improve the ratio of direct togenerally reflected sound and to otherwise discriminate against undesirable sounds.

One of the important factors in a directive sound pickup system is the solid angle over which sound is received without appreciable attenuation. This must be large enough toinclude the average area of action. At the same time the angle should be sufliciently small so that'an appreciable gain in discrimination against undesirable sounds is obtained. Another requirement is a directional characteristic which is independent of the fre- 50 quency. A system which does not possess this characteristic will produce frequency discrimination. Due to the large frequency band of the audible spectrum the use of directional systems which depend upon interference become exceedingly cumbersome and complex if an attempt is Fig. 5 illustrates the electrical connections of.-

made to obtain uniform directional characteristics.

The ribbon microphone is a simple directional pickup system in which the directional characteristics are independent of thefrequency. The ribbon microphone is an acoustic doublet, which means that the responseis a function of the cosine of the angle between the normal tothe ribbon and the direction of propagation of the incident sound. The ribbon microphone is a 5 pressure gradient microphone and its response corresponds to the velocity component of a sound wave. The combination of this'type of microphone with a microphone whose response corresponds to the pressure component of a sound wave results in the uni-directional microphone hereinafter described in connection with Figs.

The pressure gradient ribbon microphone consists of a light corrugated metallic ribbon suspended in a magnetic field and freely accessible to sound vibrations from both sides. The vibration of the ribbon due to an impressed sound wave leads to the induction of an E. M. F. correspond-, ing to the undulations of the incident sound wave.

The front and backof the ribbon are separated by an acoustic path AX. When this system is placed ina plane wave sound field a difference in pressure between the two sides of the ribbon exists due to the difierence in phase between the two sides. It is this difference in pressure-due to the difference in phase that. actuates the ribbon in the ribbon microphone.

In this analysis we will assume a plane wave sound field. Let the pressure at the front 'of the ribbon be p KcpA sin K ct-. Y

where K The Pressure at the back of the ribbon is P=KcpA sin K gt-F?) The resultant pressure on the ribbon will be the difference in pressure between the two sides and is givenv by the expression (3) Ap= 2KcpA cos (Kct) sin( 2 Thevelocity of the ribbon is given by A A (4) B0+ .4a

where ApS =the total difference in pressure acting upon the ribbon X =the reactance due to the mass of the ribbon Z =the impedance due to the air load upon the ribbon S =area of the ribbon. zAG= AG+ AG The ribbon is spaced by a few mils from the pole pieces of the magnetic. structure. This aperture gives rise to a mechanical impedance. In general the impedance due to the spacing is large compared to the mass reactance of the ribbon and may be neglected. The reactance Xac due to the mass of the ribbon and the components of the air load ZAG are shown in Fig. 6.

The generated E. M. F. induced by the motion of the ribbon is given by where B=flux density lg=length of the ribbon. S =area of ribbon.

BlgApsa from the equation is shown in Fig. 7. As will be seen the experimental results agree with the theoretically predicted response.

The above considerations have been concerned with the direction of propagation normal to the plane of the ribbon. Whenthe normal to the face of the microphone is inclined by an angle 0 to the line of propagation the acoustic path from the front to the back of the ribbon is multiplied by a factor cos 6. When a is the pressure difierence between the two sides is zero and the ribbon remains stationary. The observed directional characteristics of this microphone are shown in Fig. 8.

The velocity of the ribbon from Equation (4) can be written In this mechanical system the velocity is given where velocity P sound pressure S =area of the ribbon Z =the total mechanical impedance The generated E. M. F. induced by the motion of the ribbon is given by where B=flux density '%=leng th of the ribbon As in the case of the pressure gradient microphone the impedance due to space between the ribbon and the pole pieces may be neglected. Equation 9 shows that to maintain constant velocity in this system R must be made large compared to RAP+1.(XAP+XRP). This can be accomplished by proper choice of the resistance RL.

. ed from Equation 8 is shown in Fig. 10. As will be seen the experimental results agree with the theoretically predicted response.

A true pressure measuring instrument should not discriminate against any direction. To attain this objective in any pressure operated mi-' crophone the dimensions of the component parts 1r k (6) X 2SGKPA sin Kct sin sin (Kct+ cos I ]sin X) cos a 1/ ho-i 1204- XAG) I where must be made smallcompared to the wave length of the sound wave. ,wThis can be accomplished in I: and X +X the ribbon type of m crophone by making. the field R structure open or i well ventilated. The directional characteristics of this microphone are s betwcen the direction of s shown in Fig. 11. These results indicate that the 222:: the nm'mal the Flam the response is independent of the direction up to The phase angle between the pressure at X=O and Equation 6 above is shown in Fig. 6.

The response ofthe pressure gradient ribbon microphone described above is a measure of the velocity component in a sound wave. By a suitable modiflcation this instrument may be adapted to respond to the pressure component in a sound wave. One way in which this may be'accomplished will be described.

3000 cycles. Above' this frequency the pressure .at face of the ribbon is greater than that in free space for 0:0. This results in a slight increase in the response above this frequency and asa consequence, there is a deviation from uniform response in all directions above 3000 cycles.

In the preceding discussion we have considered two types of ribbon microphones, namely, a microphone in which the response is a measure of the velocity componentin the sound wave, and "150 By a suitable combination of the pressure microphone and the velocity or pressure gradient microphone a uni-directional microphone is produced. Such a combination is illustrated by Figures 1 to 5. This combination includes a corrugated ribbon 10 interposed between pole pieces 11 and 12 of a magnet 13 provided with a field coil 14. The ribbon 10 is supported in the magnetic field produced between the pole pieces 11 and 12, a member 15 being attached to its upper end and a member 16 being attached to its lower end. The current generated by the microphone is transmitted through terminals (not shown) connected to its opposite ends. It will be noted that the pole pieces 11 and 12 are provided with ventilating slots 17 and 18 and that a pipe or conduit 19 is provided at-its lower end with an enlarged opening which is mounted on the back of the device and covers'the upper part of the ribbon 10.

With this construction the lower part of the ribbon responds to the velocity or pressure gradient component of the sound wave and the upper part of the ribbon responds to the pressure. component of the sound wave. Theoretically, the pipe 19 should be of indefinite length to be an acoustic resistance. This, of course, is impossible. Substantially the same result is produced by a pipe filled with loosely packed felt for preventing refiections from its open end. Thus, with a pipe about three feet long the requirements of an acoustic resistance of appropriate size is obtained. As indicated by Fig. 5, the single ribbon 10 of the pressure and velocity or pressure gradient micro'phone components may be connected in series to the input transformer 20 of an amplifier 21.

r With the ribbons in the two microphones connected in series the combined generated E. M. F. is given by As will be seen from Fig. 6 the velocity of the ribbon inthe pressure gradient microphone is practically in phase with the pressure. in a plane sound wave. Figure 9 shows that the velocity of the ribbon in the pressure microphone is practically in phase with the pressure in a'plane sound wave. Therefore, if the sensitivity of the two microphones are made equal for 0:0 the resultant characteristic will be a cardioid of revolution with "E cost? the axis of revolution normal to the plane of the ribbons. This is illustrated graphically in Fig. 12. The observed directional characteristics of the combination are shown in Fig. 8. It will be seen i that these directional characteristics are practically cardioids of revolution up to 3000 cycles. Above this frequency the velocity of the ribbon in the pressure gradient microphone is slightly out of phase with the pressure. -The same is true of the pressure microphone. Therefore, we would expect some deviation from the cardioid of revolution at the higher frequencies. This is shown by the directional characteristics in Fig. 13.

The frequency characteristic of the combination for 0:0 is shown in Fig. 14.

The efiiciency of response of the uni-directional microphone in receiving sound originating in random directions will now be derived. The voltage o'utput of the uni-directional microphone for sound originating in the direction 0 is .sponsive to the pressure of said wav The output of a non-directlonai microphone for sound originating in any direction is END =2E0 This shows that the two microphones have the same sensitivity for =0.

The efliciency of energy response of the unidirectional microphone for sounds originating in random directions, all directions being equally probable. is

uni-directional microphone can be used at 1.7

the distance of a non-directional microphone.

The large solid angle -over which this microphone receives sound without appreciable attenuation indicates that practically any action can be covered with a single microphone.

Referring to Fig. 12 it will be seen that for angles larger than the response is relatively small. In general, undesirable sounds such as camera noise will fall in this region. Therefore, the particular directional characteristics exhibited by this microphone will be found very useful in overcoming undesirablesounds in sound motion picture recording or broadcast sound. pickup, where desired sounds originate in front and undesired sounds generally to the rear of the microphone.

Having thus described our invention, what we.

which is responsive to the velocity component of i a sound wave and another part of which is re- 3. The combination of means for producing a magnetic field, an elongated conductor mounted in said field, and means arranged to render only a part of said conductor responsive to the pressure gradient of a sound wave. b

4. The combination of means for producing a magnetic field, an elongated conductor mounted in said field, and means including a pipe in close proximity to a part of said conductorfor rendering said part responsive to the pressure of a sound'wave.

5. The combination of means for producing a magnetic field, an elongated conductor mounted in said field, and means including a pipe containing an anti-reflecting material and in close proximity to a part of said conductor for rendering said part responsive to the pressure of a sound wave. I

6. The combination of means for producing a magnetic field, an elongated conductor mounted in said field, and means including a pipe loosely I packed with felt and in close proximity to a part '7. The combination of means 101- producing a magnetic field, an elongated conductor mounted in said field, and an acoustic means adjacent a part of said conductor for rendering said part responsive to the pressure of a sound wave:

8. The combination of means for producing a. magnetic field, an elongated conductor mounted in said field, means arranged to render only a part of said conductor responsive to the pressure of a sound wave, and electrical amplifying means connected to the opposite ends of said conductor.

HARRY F. OLSON. JULIUS WEINBERGER. 

